Preparation Of Micro-Arc Oxidation Coatings On Surface Of 5a06 Aluminium Alloy/Galvanized Q235 Steel Welding-Brazing Joint And Research On Its Static Corrosion Resistance To LBE

Lead-bismuth eutectic alloy(LBE)has the characteristics of high density,low neutron activity,high specific heat capacity,and has been considered by the international community to be an important candidate for the future accelerator subcritical driving system(ADS)and lead-cooled fast reactor.Metal structures that have been exposed to liquid LBE for a long time are subject to corrosion by liquid LBE.The micro-arc oxidation ceramic coatings are widely used because of its characters,like high hardness,good wear resistance,high temperature resistance,corrosion resistance,etc.Modifying the surface of metallic material by micro-arc oxidation technology can significantly improve the performance and service life of metal materials.In this paper,micro-arc oxidation ceramic coatings were prepared on galvanized Q235 steel considered as substrate,and the corrosion resistance of ceramic coatings to liquid LBE was mainly studied in order to provide a theoretical study for the application of micro-arc oxidation ceramic coatings prepared on other nuclear structural material surfaces.In this paper,TIG welding-brazing was used to form 5A06 aluminum matrix intermediate layer on surface of galvanized Q235 steel.An excellent welded joint was obtained by changing welding current and wire feeding speed,and a proper post-weld heat treatment was performed.Micro-arc oxidation treatment was performed on weld surface of heat-treated joint,and the impacts of pulse frequency,current density,duty cycle,and oxidation time on surface morphology,thickness,roughness and phases of ceramic coatings formed on surface of weld seam were investigated.Through analysis of microstructure and properties of joints and ceramic coatings,the best welding parameters and micro-arc oxidation process parameters were found.The formation of fine-performance ceramic coatings on weld surface can improve corrosion resistance of metal materials in high-temperature liquid LBE.The conclusions are as follows:The dissimilar metal welded joint between aluminum and steel obtained by using AlSi5 welding wire will generate intermetallic compound layer.The compositions and thickness of the compound layer will affect performance of welded joint.Phase analysis results show that the intermetallic compound layer is mainly composed of Al8Fe2Si phases and a small amount of[Al,Fe,Si]phases.The welding current has an impact on thickness of the intermetallic compound layer.With the increasing of welding current,the thickness of the intermetallic compound layer increases obviously.However,the compositions do not change much.With the increasing of wire feeding speed,the thickness of the intermetallic compound layer decreases.After the post-weld heat treatment of welded joint was conducted,the grains in weld seam become fine,the tensile strength increases,and the thickness of the compound layer can increase significantly(<10 ?m),but the phases of the intermetallic compound layer do not change significantly.According to microstructure and performance of welded joint,the recommended parameters are that the welding current is 90 A,the wire feeding speed is 0.34 m min-1,and the heat treatment process is 280? for 30 min.The morphology,phases,thickness and roughness of ceramic coatings will be effected by micro-arc oxidation process parameters.The ceramic coatings are composed of compact a-Al2O3 phases and loose y-Al2O3 phases.Meanwhile,there are some discharge pores on surface of ceramic coatings.As the pulse frequency increases,the content of a-Al2O3 phases,aperture of discharge pores,thickness and roughness of ceramic coatings will decrease,however,those of ceramic coatings will increase with the augment of the current density,oxidation time and duty cycle,but the variation of parameters does not bring about obvious changes in compositions and phases of ceramic coatings.When the pulse frequency is 500 Hz,the current density is 10 A/dm2,the oxidation time is 30 min,and the duty cycle is 4%,the thickness of ceramic coatings is moderate and the surface is smooth.Meanwhile,discharge micropores on surface are fine.Specimens used to conduct corrosion test are Q235 steel,non-micro-arc oxidation treated weld seam and micro-arc oxidation treated weld seam.The results show that Q235 steel can be mainly dissolved in static liquid Pb-Bi alloy.The surface of Q235 steel and specimen without micro-arc oxidation have some obvious corrosion pits.Some Pb and Bi elements diffuse into 5A06 aluminum matrix intermediate layer of specimen without micro-arc oxidation.The surface morphology of micro-arc oxidation ceramic coatings does not change significantly.In addition,new phases can not be found on surface.Only a small amount of Pb and Bi elements exist in outer layer of ceramic coatings,but Pb and Bi elements can not be found in 5A06 aluminum matrix intermediate layer and steel matrix.So the existence of ceramic coatings has excellent protective effect on steel substrate.Corrosion resistance of ceramic coatings on 5A06 aluminum matrix intermediate is better than that of Q235 steel.